The present invention relates generally to magnetron sputtering apparatus, and more particularly to rotating magnetrons.
DC reactive sputtering is the deposition process most often used for large area commercial coating applications, such as the application of thermal control coatings to architectural and automobile glazing. In this process, the articles to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. This system may be referred to as a continuous in-line system or a glass coater.
Inside the vacuum chambers, a sputtering gas discharge is maintained at a partial vacuum at a pressure of about 3 millitorr. The sputtering gas comprises a mixture of an inert gas, such as argon, with a small proportion of a reactive gas, such as oxygen, for the formation of oxides.
Each chamber contains one or more cathodes held at a negative potential of about -200 to -1000 volts. A layer of material to be sputtered is applied to the cathode surface. This surface layer is known as the target or the target material. The reactive gas forms the appropriate compound with the target material.
Ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, are deposited on a substrate, such as a glass sheet, passing beneath the target. The atoms react on the substrate with the reactive gas in the sputtering gas discharge to form a thin film. It is advantageous to produce the gas discharge in the presence of a magnetic field using an apparatus known as a magnetron. An array of magnets is mounted in a fixed position behind the target. The magnetic field causes electrons from the discharge to be trapped in the field and travel in a spiral pattern, which creates a more intense ionization and higher sputtering rates. Appropriate water cooling is provided to prevent overheating of the target.
The architectural glass coating process was made commercially feasible by the development of the magnetically-enhanced planar magnetron. A disadvantage of the planar magnetron is that the target material is only sputtered in the narrow zone defined by the magnetic field. This creates a "racetrack"-shaped sputtering zone on the target. Thus, a "racetrack"-shaped erosion zone is produced as sputtering occurs. This causes a number of problems. For example, localized high temperature build-up eventually limits the power at which the cathodes can operate, and only about 25 percent of the target material is actually used before the target must be replaced.
The rotary or rotating magnetron was developed to overcome some of the problems inherent in the planar magnetron. The rotating magnetron uses a substantially cylindrical cathode and target. The cathode and target are rotated continually over a magnetic array which defines the sputtering zone. As such, a new portion of the target is continually presented to the sputtering zone which eases the cooling problem, allowing higher operating powers. The rotation of the target also ensures that the erosion zone comprises the entire circumference of the cathode covered by the sputtering zone. This increases target utilization. The rotating magnetron is described further in U.S. Pat. Nos. 4,356,073 and 4,422,916, the entire disclosures of which are hereby incorporated by reference.
The rotating magnetron requires bearings to permit rotation, and vacuum seals for the drive shaft, the electrical conduits and the cooling conduits. Vacuum and rotary water seals have been used to seal around the drive shaft and the conduits which extend between the coating chamber and the ambient environment. However, such seals have a tendency to develop leaks under conditions of high temperature and high mechanical loading. Various mounting, sealing and driving arrangements are described in U.S. Pat. Nos. 4,443,318; 4,445,997; and 4,466,877, the entire disclosures of which are also hereby incorporated by reference. These patents describe rotating magnetrons mounted horizontally in a coating chamber and supported at both ends. In this arrangement, two spindles, one of which is a drive shaft and the other an idler, are attached to the ends of the cathode.
It is often preferable, however, to support the magnetron at only one end designated as the drive end by a cantilever mount. The other end of the cathode may be referred to as the free end. The cantilever mounting arrangement produces the highest bearing loads. Several examples of cantilever mounted rotary magnetrons are given in Design Advances and Applications of the Rotatable Magnetron, Proceedings of the 32nd National Symposium of the American Vacuum Society, Vol. 4, No. 3, Part 1, pages 388-392 (1986), the entire text of which is hereby incorporated by reference. A cantilever mounted magnetron usually includes a bearing housing containing a drive shaft, a rotary vacuum seal, and at least two bearings spaced along the drive shaft, one of which may function as a shaft seal.
A rotating magnetron incorporating a cantilever mounted removable cathode and having low vacuum seal loads is described in U.S. Pat. Nos. 5,100,527 and 5,200,049, both assigned to the owner of the subject application, the entire disclosures of which are hereby incorporated by reference. Such a system allows, among other things, the cathode to be removed easily and without special equipment, thus reducing system down time both by reducing the time required to replace a cathode and by making simultaneous removal of two or more cathodes practical.
In general, a dark space shield or sleeve may be concentrically disposed about the cathode body and spaced from its surface to form a gap. The distance across this gap is less than the dark space length. The dark space is the region of gas discharge next to the cathode. Here, the electrons accelerate under an applied operating voltage to become adequately energized to cause ionization of the sputtering gas. The dark space length is a function of the type of sputtering gas, the gas pressure and the applied electric field. The dark space length, for example, may be on the order of three millimeters.
The dark space shield protects the cathode body from the gas discharge and resultant ion bombardment. Dark space shields are usually provided at both the drive end and the free end of the cathode. The shield around the drive end of the cathode body should prevent the sputtering gas discharge from contacting that end. The dark space shield has a provision such a flange, for attachment to an appropriate mounting surface. For the dark space shields used heretofore at the supported end or ends of the cathode, the mounting surface has been the chamber wall or a flange attached to the chamber wall, such that the dark space shield does not rotate with the cathode. The shield is also electrically insulated from this mounting surface so that it is electrically isolated therefrom. Thus, it floats electrically and acquires an electrical potential of the gas discharge. A preferred material for the shield is stainless steel.
During sputtering, a film of deposited material grows on the dark space shield, usually under tensile or compressive stress. The stress is highest on sharp edges. Eventually the film deposited begins to spall off, beginning usually on such sharp edges and on areas where the film is thickest. If the resulting flakes of material fall onto a substrate, they obstruct deposition on the areas of the substrate that they cover, resulting in defective products. In order to minimize the rate of film growth on a given dark space shield surface, that surface should point in a direction as close as possible to the direction away from the sputtering target.
The spacing between the dark space shield and the cathode must be well controlled to be less than a dark space length and to ensure that the shield does not touch the cathode. In the rotating magnetron systems known heretofore, the imprecision in the positioning of the dark space shield around a supported end of the cathode can arise from four different sources. These sources are the imprecision in the positioning of the shield with respect to its mounting surface of the chamber wall, the imprecision in the positioning of the bearing housing relative to the chamber wall, the imprecision in the positioning of the spindle relative to the bearing housing, and the imprecision in the positioning of the cathode relative to the spindle.
Accordingly, an object of the present invention is to provide a dark space shield for a supported end of a rotatable cathode wherein the imprecision in the shield to cathode spacing is reduced.
Another object of the present invention is to provide a dark space shield wherein the tendency of the growing film to spall off the shield is reduced.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.